linux-kernel - Re: [FYI PATCH] Revert "KVM: x86/mmu: Zap only TDP MMU leafs in kvm_zap_gfn

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Message-ID: <Yj4kjRSuBDfupRqW@google.com>
Date:   Fri, 25 Mar 2022 20:22:37 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Subject: Re: [FYI PATCH] Revert "KVM: x86/mmu: Zap only TDP MMU leafs in
 kvm_zap_gfn_range()"

On Fri, Mar 25, 2022, Paolo Bonzini wrote:
> On 3/25/22 00:57, Sean Christopherson wrote:
> > Can I have 1-2 weeks to try and root cause and fix the underlying issue before
> > sending reverts to Linus?  I really don't want to paper over a TLB flushing bug
> > or an off-by-one bug, and I really, really don't want to end up with another
> > scenario where KVM zaps everything just because.
> 
> Well, too late...  I didn't want to send a pull request that was broken,

Ah, I didn't see that it was in the initial pull request, thought it was only in
kvm/next.  I'll send a full patch.

> Mingwei provided a convincing reason for the breakage.

No, the side effects are completely benign, and arguably desirable.  The issue is
that KVM loses a pending TLB flush if there are multiple roots.